Generate digital signature for text file in java - study materials for competitive exams

import java.awt.*;
import java.io.*;
import java.security.*;
import java.awt.event.*;
import java.io.File;
import javax.swing.*;
public class frame1 extends JFrame
{
/**
*
*/
private static final long serialVersionUID = 1L;
private JButton Browse,gen;
private JTextField path,comm1;
private Container container;
public static void main (String[] args)
{
frame1 application = new frame1();
application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
application.setResizable(false);
}
public frame1()
{
super("Generate Signature");

container=getContentPane();
setLayout(null);
path = new JTextField(30);
container.add(path);
path.setBounds(30, 20, 150, 25);
Browse = new JButton("Browse");
container.add(Browse);
Browse.setBounds(190, 20, 80, 25);
gen = new JButton("Generate");
container.add(gen);
gen.setBounds(30, 70, 100, 25);
comm1=new JTextField(50);
container.add(comm1);
comm1.setBounds(30,100,250,25);
comm1.setEditable(false);
String ans5 = "Project by: Akshay Tikekar & Kunal Kumar";
comm1.setText(ans5);
Browse.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent event)
{

JFileChooser chooser = new JFileChooser();
chooser.showOpenDialog(null);
File file = chooser.getSelectedFile();
final String filenm = file.getPath();
path.setText(filenm);

gen.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent event)
{
try{

/* Generate a key pair */

KeyPairGenerator keyGen = KeyPairGenerator.getInstance("DSA", "SUN");
SecureRandom random = SecureRandom.getInstance("SHA1PRNG", "SUN");

keyGen.initialize(1024, random);

KeyPair pair = keyGen.generateKeyPair();
PrivateKey priv = pair.getPrivate();
PublicKey pub = pair.getPublic();

/* Create a Signature object and initialize it with the private key */

Signature dsa = Signature.getInstance("SHA1withDSA", "SUN");

dsa.initSign(priv);

/* Update and sign the data */

FileInputStream fis = new FileInputStream(filenm);
BufferedInputStream bufin = new BufferedInputStream(fis);
byte[] buffer = new byte[1024];
int len;
while (bufin.available() != 0) {
len = bufin.read(buffer);
dsa.update(buffer, 0, len);
};

bufin.close();

/* Now that all the data to be signed has been read in,
generate a signature for it */

byte[] realSig = dsa.sign();

/* Save the signature in a file */
FileOutputStream sigfos = new FileOutputStream("signature");
sigfos.write(realSig);

sigfos.close();

/* Save the public key in a file */
byte[] key = pub.getEncoded();
FileOutputStream keyfos = new FileOutputStream("publickey");
keyfos.write(key);

keyfos.close();

} catch (Exception e) {
System.err.println("Caught exception " + e.toString());
}

}});
}});
setLocation(350,250);
setSize(300,200);
setVisible(true);
}
}

/* NOTE :-Save the code as " frame1.java " and give only text(.txt) file as argument.Signature will be created where frame1.java will be saved. */

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